So I just bought a 91 rx7 and I'm getting a vibration in the drivetrain when starting off in first gear, it decreases once car is moving. A little research on mazdatrix has lead me to believe its the front u-joint.

Has anyone else experienced this problem and was the front u-joint to blame?

 

I'd expect to see vibration across the board with a bad u-joint, that would increase and decrease based on RPM changes.

Just go under the car and try to tear the shaft off of the front u-joint. I think there are heat shields in the way which makes this a bit annoying. But you should be able to feel any play.

Also check the engine (especially passenger) mounts and rear transmission mount.

 

I have experienced the kind of shaking the OP describes, and it did turn out to be the front U-joint. Apparently the shaking will only be constant if it's the rear U-joint that has failed.

 

Thanks for the replies guys. I'll have to get it up on a lift and check it out.

 

So I crawled underneath the car yesterday and found what looks to me like a modified transmission cross member. The previous owner said the car had a new transmission which I'm guessing meant he got an S4 transmission and modified the cross member by welding the two cross members together.

The problem is the car vibrates like hell when starting out. I took some pics this morning of the cross member. Looks to me like the one up against the transmission has the middle bushing busted out. Any idea what I need to do to fix this?

My guess is replace the s4 cross member (the one with the busted middle bushing) and replace the trans mounts.

 

Your car was originally an automatic. There's a write-up with pic here that shows a similar method that's been used: http://www.rotaryresurrection.com/2n...to_5speed.html.

In your first picture, it definitely looks like the bushing is wasted. There shouldn't be a big gap between the crossmember and the nut on the end there. The problem is you'll have to buy a whole new crossmember, then have it welded up just like that one. Or you could do more like they did in the write-up above, and use a piece of metal with no bushing. The advantage would be no bushing to break, but you may find a little more vibration from the drivetrain.

 

Ah so that's why its got the modified cross member. Should get a s4 or s5 cross member to replace the one with the missing bushing?

 

If the previous owner was right, an S4 NA transmission cross-member would be the right one. It looks to me like it may be an S4 due to the style of the mount. I've attached a couple pics of my TII transmissions. The one on the left is S4, and the one on the right is S5. The close-up in the first pic is of the S4. It has 2 bolts holding the mount on from the bottom of the tail housing. For S5s, they switched the mount style to one that has 4 bolts (2 per side) that hold it onto the sides of the tail housing.

 

Those pics help a lot. My transmission is an S5 it has the 4 holes for the Y bracket to mount into it. I think they just rigged it with an incorrect bracket. To fix it right I'll need the S5 cross member, tranny mounts and the Y then have someone weld it to the automatic tranny cross member. I wish people would do things right the first time around.

Is there a way to tell whether the tranny the swapped in is a N/A or Turbo?

 

You have an NA transmission. That also appears to be leaking.

I hate seeing stuff like that bracket. What a horrible way to do something. Whoever did it doesn't understand a thing about leverage and stresses. I'd honestly recommend scrapping that bracket and making something that doesn't suck, if you have the equipment.

You are also missing the exhaust heat shields.

 

Yeah when I first saw the bracket I couldn't believe someone thought that it was a good idea to have two flat plates an 1/8 of an inch thick holding the tranny in place. Classes are starting up again in a couple days and I can use the machine shop there, hopefully I can make something myself that will work or find a machine shop that can make it. The U channel adapter they use on the rotary resurrection site seems very doable and will actually provide some vertical stability.

 

I finally got this problem fixed. Turns out the tranny in the car is an s4 manual the problem was that the s4 manual tranny cross member had a blown out bushing/spacer. I made an adapter out of an 8 inch piece of 4 inch C channel (according to the rotatory resurrection specs) I picked up at a junk yard and used a new automatic cross member+my adapter+the old auto spacer+ some washers and a ~3inch grade 8 bolt to fix the problem. I still have some vertical movement when accelerating in first. I'm guess this is because the tranny's weight is all on the auto cross member spacer/bushing. I'm thinking I'll get the adapter welded to the cross member and the vibration will decrease.

Now I just need to fix my tranny leak and the OMP leak, along with a bunch of other miscellaneous crap.